Structural determinants of vascular resistance properties in hypertension. Haemodynamic and model analysis.

The average internal radius (ri) of the resistance vessels of the hindquarter (HQ) bed was narrower in renovascular and genetic hypertension than in normotensive controls. The narrowing was approximately uniform over the full range of vasomotor tone, which accounted for the bed's property as an 'amplifier' of vascular resistance (R) (increased slope (S) of the dose-R response curve) and for the elevated R at maximum dilatation (Rmin). In the model we examined the effects on the dose-R curve parameters of altering wall/internal radius (w/ri) ratio, ri and wall 'stiffness' one at a time, whilst the others were held constant: only narrowing of ri led to increases in both S and Rmin; with hypertrophy alone, S increased but Rmin was reduced, whilst increased wall stiffness increased Rmin but lowered S. Thus, for hypertrophy to be associated with rises of both S and Rmin, it must be linked to lumen narrowing, to increased wall stiffness, or to both. Preferential deposition of new material towards the lumen will link hypertrophy to narrowing. It has been suggested that narrowing can occur without hypertrophy ('remodelling'). In the model an increase of only 1-2% WV was required to produce rises in w/ri of congruent to 30-50% when associated with congruent to 10-15% reduction in ri, which is close to the limit of detection. From the literature, the sites of greater narrowing in hypertension extend down to small arteries and large arterioles. The rise in BP upstream from those sites, due to the vascular amplifier, offsets the down-stream effects of vascular narrowing on blood flow and this negative feedback system helps to maintain elevation of BP at a stable level. We also examined developmental rise in R between 4 and 50 weeks, which affected SHR and WKY in the same proportion: structural factors (vascular length of larger arteries, 'rarefaction' of arterioles and capillaries) accounted for only about half the rise in R, and the remainder was probably due to developmental changes in muscle function.